1. Field of the Invention
This invention concerns a lamp unit used as a light source for projection equipment that uses a digital micro-mirror device.
2. Description of Related Art
Projection equipment is required to project an image on a screen, evenly and with full color characteristics. To achieve this, metal halide lamps, in which a mercury and a metal halide are sealed, are commonly used as light sources. In recent times these metal halide lamps have come to have very small inter-electrode gaps, making them smaller and more nearly point light sources.
Lamps with unprecedentedly high mercury vapor pressure, 200 bar (about 197 atmospheres), have recently been proposed to replace metal halide lamps. Making the mercury vapor pressure higher is intended to suppress the spread of the arc and make the light output even higher. Examples include JPO Kokai Patents H2-148561 (U.S. Pat. No. 5,109,181) and H6-52830 (U.S. Pat. No. 5,497,049).
In liquid crystal projection equipment, the light radiated from the light source is divided into three colors (R, G, B). Each ray is then adjusted by the liquid crystal, and the three colors are combined and projected onto the screen.
In liquid crystal projection equipment of this sort, the previously mentioned super high pressure lamp, surrounded by a reflector mirror, is used as the light source. The reflector mirror that illuminates the liquid crystal panel can be parabolic or elliptical, but is generally a prolate ellipsoid with a short focal distance.
Recently, however, projection equipment that uses DMD (Digital Micromirror Devices) instead of liquid crystal has been proposed. In particular, DMD substrates have been realized in the small projector industry. In these small projectors, the light radiated from the light source passes through a three-color (R, G, B) filter and illuminates the DMD, and the light reflected from the DMD shines on the screen. The DMD is packed with millions of small mirrors, one per pixel, and the direction in which light is reflected can be changed by controlling the orientation of each mirror independently.
This DMD substrate projection (DLP) equipment, because it does not require the RGB three-color liquid crystal panel, can be made smaller than the liquid crystal projection equipment (down to about B5 size); and in that sense is quite remarkable.
The projection equipment using DMD has a great advantage in that the equipment as a whole is quite small. Thus, it is also necessary to make the distance between the lamp and the rotating filter as small as possible. Moreover, in order to confine the light reflected from the lamp toward the DMD, an elliptical, beam-condensing mirror is used instead of a parabolic mirror. That is, because of the size constraints, the reflected light from the lamp has to be concentrated in a short distance and a short focal length, elliptical, beam-condensing mirror is adopted as the reflector mirror.
Moreover, in the event that an alternating current discharge lamp is used as the light source, the changes in the lamp polarity have to be synchronized with the movement of the rotating filter and the DMD, and thus fluctuation of the reflected light with each change of polarity need be prevented. For these reasons, it is advantageous to use a direct current discharge lamp rather than an alternating current discharge lamp, as the light source for DMD projection equipment.
In this way, the necessary conditions for the light source of projection equipment using DMD include the use of a short focal-length, elliptical, beam-condensing mirror as the reflector mirror, and a direct current, very high pressure, short-arc mercury lamp as the discharge lamp.
However, there is the problem that the light radiated from the lamp that cannot be accommodated by the rotating filter is reflected by the filter and illuminates the seals of the lamp, and causes the seals to heat up. Since an elliptical reflector mirror is used, as stated above, the arcing point of the discharge lamp is located at the first focal point of the ellipse, and the second focal point is the beam-condensing point of the rotating filter. Because DMD projection equipment uses this sort of short focal length and an elliptical, beam-condensing mirror, the distance between the rotating filter and the seal is short; overheating is a problem. Thus, when the temperature of the lamp seal rises, a major problem such as the breakage, oxidation or melting of the metallic foil within the seal can occur.
Secondly, a direct current discharge lamp produces different amounts of heat at the anode and the cathode, and generally the volume of the anode is larger than that of the cathode, in connection with heat capacities. If miniaturization of the lamp causes the anode to be larger, cool regions are liable to form near the base of the larger anode. The arc lamps used in projection equipment frequently include more than 0.15 mg/mm3 of mercury, and have a characteristic problem of non-vaporized mercury accumulating in these cool regions. This non-vaporized mercury is a problem, since it obstructs the generation of the light spectrum that is desired.
The problem to be resolved by this invention is the provision of a lamp unit suitable for projection DMD equipment.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the embodiments.
The embodiments of the invention described below in detail, with reference to the accompanying figures.